1. Field of the Invention
This invention relates generally to apparatus and processes using fluidized beds. More specifically, this invention relates to increasing the lateral mixing of particles in fluidized beds.
2. Description of the Prior Art
Fluidized beds are used in many industrial applications. One use in particular is in the regenerator of a petroleum refining process.
Fluid catalytic cracking (FCC), as well as Resid FCC(RFCC), is a catalytic conversion process for cracking heavy hydrocarbons into lighter hydrocarbons by bringing the heavy hydrocarbons into contact with a catalyst composed of finely divided particulate material. Most FCC units use zeolite-containing catalyst having high activity and selectivity.
The basic components of the FCC reactor section include a riser, a reactor, a catalyst stripper, and a regenerator. In the riser, a feed distributor inputs the hydrocarbon feed which contacts the catalyst and is cracked into a product stream containing lighter hydrocarbons. Catalyst and hydrocarbon feed are transported upwardly in the riser by the expansion of the lift gases that result from the vaporization of the hydrocarbons, and other fluidizing mediums, upon contact with the hot catalyst. Steam or an inert gas may be used to accelerate catalyst in a first section of the riser prior to or during introduction of the feed. Coke accumulates on the catalyst particles as a result of the cracking reaction and the catalyst is then referred to as spent catalyst. The reactor disengages spent catalyst from product vapors. The catalyst stripper removes absorbed hydrocarbon from the surface of the catalyst. The regenerator removes the coke from the catalyst and recycles the regenerated catalyst into the riser.
The spent catalyst particles are regenerated before catalytically cracking more hydrocarbons. Regeneration occurs by oxidation of the carbonaceous deposits to carbon oxides and water. The spent catalyst is introduced into a fluidized bed at the base of the regenerator, and oxygen-containing combustion air is passed upwardly through the bed. After regeneration, the regenerated catalyst is returned to the riser.
Oxides of nitrogen (NOx) are usually present in regenerator flue gases but should be minimized because of environmental concerns. Regulated NOx emissions generally include nitric oxide (NO) and nitrogen dioxide (NO2), but the FCC process can also produce N2O. In an FCC regenerator, NOx is produced almost entirely by oxidation of nitrogen compounds originating in the FCC feedstock and accumulating in the coked catalyst. At FCC regenerator operating conditions, there is negligible NOx production associated with oxidation of N2 from the combustion air. Production of NOx is undesirable because it reacts with volatile organic chemicals and sunlight to form ozone.
The two most common types of FCC regenerators in use today are a combustor-style regenerator and a bubbling bed regenerator. Bubbling bed and combustor-style regenerators may utilize a CO combustion promoter comprising platinum for accelerating the combustion of coke and CO to CO2. The CO promoter decreases CO emissions but increases NOx emissions in the regenerator flue gas.
The combustor-style regenerator has a lower vessel called a combustor that burns nearly all the coke to CO2 with little or no CO promoter and with low excess oxygen. The combustor is a highly backmixed fast fluidized bed. A portion of the hot regenerated catalyst from the upper regenerator is recirculated to the lower combustor to heat the incoming spent catalyst and to control the combustor density and temperature for optimum coke combustion rate. As the catalyst and flue gas mixture enters the upper, narrower section of the combustor, the velocity is further increased and the two-phase mixture exits through symmetrical downturned disengager arms into an upper regenerator. The upper regenerator separates the catalyst from the flue gas with the disengager arms followed by cyclones and return it to the catalyst bed which supplies hot regenerated catalyst to both the riser reactor and lower combustor.
A bubbling bed regenerator carries out the coke combustion in a dense fluidized bed of catalyst. Fluidizing combustion gas forms bubbles that ascend through a discernible top surface of a dense catalyst bed. Only catalyst entrained in the gas exits the reactor with the vapor. Cyclones above the dense bed separate the catalyst entrained in the gas and return it to the catalyst bed. The superficial velocity of the fluidizing combustion air is typically less than 1.2 m/s (4 ft/s) and the density of the dense bed is typically greater than 480 kg/m3 (30 lb/ft3) depending on the characteristics of the catalyst. The mixture of catalyst and vapor is heterogeneous with pervasive vapor bypassing of catalyst. The temperature will increase in a typical bubbling bed regenerator by about 17° C. (about 30° F.) or more from the dense bed to the cyclone outlet due to combustion of CO in the dilute phase. The flue gas leaving the bed may have about 2 mol-% CO. This CO may require about 1 mol-%. oxygen for combustion. Assuming the flue gas has 2 mol-% excess oxygen, there will likely be 3 mol-% oxygen at the surface of the bed and higher amounts below the surface. Excess oxygen is not desirable for low NOx operation.
Refiners often use CO promoter (equivalent to 0.5 to 3 ppm Pt inventory) to control afterburn at the low excess O2 required to control NOx at low levels. While low excess O2 reduces NOx, the simultaneous use of Pt CO promoter often needed for after-burn control can more than offset the advantage of low excess O2.
Bubbling bed regenerators have a fluidized bed. Fluidized beds generally mix well vertically, up and down, but not laterally, or horizontally. Rising bubbles draw catalyst up with term in their wakes and the catalyst constitutes about one third of total bubble volume. This is the principle solids mixing mechanism in fluidized beds. In a bubbling bed, also known as a dense catalyst bed, combustion gas forms bubbles that ascend through a discernible top surface of a dense catalyst bed. Relatively little catalyst is entrained in the combustion gas exiting the dense bed. These bubbles rise with little horizontal displacement.
The superficial velocity of the combustion gas is typically less than 1.2 m/s (4.2 ft/s) and the density of the dense bed is typically greater than 640 kg/m3 (40 lb/ft3) depending on the characteristics of the catalyst. The mixture of catalyst and combustion gas is heterogeneous with pervasive gas bypassing of catalyst.
The dilute transport flow regime is typically used in FCC riser reactors. In transport flow, the difference in the velocity of the gas and the catalyst is relatively low with little catalyst back mixing or hold up. The catalyst in the reaction zone maintains flow at a low density and very dilute phase conditions. The superficial gas velocity in transport flow is typically greater than 2.1 m/s (7.0 ft/s), and the density of the catalyst is typically no more than 48 kg/m3 (3 lb/ft3). The density in a transport zone in a regenerator may approach 80 kg/m3 (5 lb/ft3). In transport mode, the catalyst-combustion gas mixture is homogeneous without gas voids or bubbles forming in the catalyst phase.
Intermediate of dense, bubbling beds and dilute transport flow regimes are turbulent beds and fast fluidized regimes. In a turbulent bed, the mixture of catalyst and combustion gas is not homogeneous. The turbulent bed is a dense catalyst bed with elongated voids of combustion gas forming within the catalyst phase and a less discernible surface. Entrained catalyst leaves the bed with the combustion gas, and the catalyst density is not quite proportional to its elevation within the reactor. The superficial combustion gas velocity is between about 1.1 and about 2.1 ml/s (3.5 and 7 ft/s), and the density is typically between about 320 and about 640 kg/m3 (20 and 40 lb/ft3) in a turbulent bed.
Fast fluidization defines a condition of fluidized solid particles lying between the turbulent bed of particles and complete particle transport mode. A fast fluidized condition is characterized by a fluidizing gas velocity higher than that of a dense phase turbulent bed, resulting in a lower catalyst density and vigorous solid/gas contacting. In a fast fluidized zone, there is a net transport of catalyst caused by the upward flow of fluidizing gas. The catalyst density in the fast fluidized condition is much more sensitive to particle loading than in the complete particle transport mode. From the fast fluidized mode, further increases in fluidized gas velocity will raise the rate of upward particle transport, and will sharply reduce the average catalyst density until, at sufficient gas velocity, the particles are moving principally in the complete catalyst transport mode. Thus, there is a continuum in the progression from a fluidized particle bed through fast fluidization and to the pure transport mode. The superficial combustion gas velocity for a fast fluidized flow regime is typically between about 1.5 and about 3.1 m/s (5 and 10 ft/s) and the density is typically between about 48 and about 320 kg/m3 (3 and 20 lb/ft3).
A combustor-style regenerator is a type of regenerator that completely regenerates catalyst in a lower, first combustion chamber under fast fluidized flow conditions with a relatively small amount of excess oxygen. A riser carries regenerated catalyst and spent combustion gas to a separation chamber wherein significant combustion occurs. Regenerated catalyst in the separation chamber is recycled to the lower combustion phase to heat the spent catalyst about to undergo combustion. The regenerated catalyst recycling provides heat to accelerate the combustion of the lower phase of catalyst. Combustor-style regenerators are advantageous because of their efficient oxygen requirements.
As greater demands are placed on FCC units, combustor vessels are being required to handle greater catalyst throughput. Greater quantities of combustion gas are added to the combustor vessels to combust greater quantities of catalyst. As combustion gas flow rates are increased, so does the flow rate of catalyst between the combustion and separation chamber increase. Hence, unless the combustion chamber of a combustor vessel is enlarged, the residence time of catalyst in the lower zone will diminish, thereby decreasing the thoroughness of the combustion that must be achieved before the catalyst enters the separation chamber.
An enlarged first chamber diameter increases the diameter of the fluidized bed and therefore the distance between the spent catalyst, at a cooler temperature, input and recycled catalyst, at a hotter temperature, is increased. Areas of temperature difference and generally stagnant zones of the high oxygen concentrations and may result and combustion efficiency may decrease. In the first chamber vertical mixing may occur, but there is usually little horizontal, or lateral, mixing. There exists a need for better lateral mixing in fluidized beds.